Recently, attention has been paid to high-output fiber lasers that utilize optical fibers doped with rare earth element. A high-output fiber laser is constituted such that pump light and signal light propagate in the fiber while amplifying the signal light using the pump light. Such a fiber has advantages, for example, in that the fiber is easily cooled, and it is possible to reduce the size of an apparatus.
In such a high output fiber laser, a fiber having an effect of a wavelength filter is required so as to suppress non-linear effects such as amplified spontaneous emission (ASE) and stimulated Raman scattering.
For example, a technique described in the Patent Reference 1 has been proposed as a conventional art related to an optical fiber that functions as a wavelength filter.
This prior art discloses that a wavelength filtering effect is provided such that the mode propagating in a normal core having a refractive index larger than the clad is coupled with the mode propagating in a portion of similarly increased refractive index provided in a periphery of the core only at a specific wavelength.
Patent Reference 1: United States Patent Application, Publication No. 2006/257071.